1. Field of the Invention
The present invention relates to a liquid density sensor, and more particularly to a high accuracy liquid density sensor.
2. Description of Related Art
Generally, a high-pressure liquid storage device usually comprises a liquid density sensor to sense a density of liquid stored in the high-pressure liquid storage device.
With reference to FIG. 8, a conventional liquid density sensor comprises a sensing module 50, a sensing rod 60, a first floating ball 70 and a second floating ball 80.
The sensing module 50 has a controller 51 having computing functions.
The sensing rod 60 comprises a hollow tube having two opposite openings and a sensing line 61 mounted in the hollow tube, wherein one end of the sensing line 61 is connected to the controller 51 of the sensing module 50. The first floating ball 70 has a housing 71, a floating part 72, a first magnetic unit 73 and a second magnetic unit 74. The housing 71 has a first opening 711, wherein the sensing rod 60 is mounted through the first opening 711. Thus, the first floating ball 70 is sleeved on the sensing rod 60 and can move in an axial direction of the sensing rod 60. The floating part 72 is mounted in the housing 71 and near the sensing module 50 such that the first floating ball 70 floats on a surface of the liquid. The first magnetic unit 73 and the second magnetic unit 74 are mounted in the housing 71 and around the sensing rod 60, wherein the first magnetic unit 73 is proximal to the floating part 72, and the second magnetic unit 74 is distal from the floating part 72.
The second floating ball 80 is mounted in the housing 71 of the first floating ball 70 and around the sensing rod 60, and comprises a second opening 81 and a third magnetic unit 82. The second floating ball 80 can move in the axial direction of the sensing rod 60. The third magnetic unit 82 is mounted around the sensing rod 60 and between the first magnetic unit 73 and the second magnetic unit 74. Two magnetic poles of the third magnetic unit 82 respectively face the first magnetic unit 73 and the second magnetic unit 74, wherein the third magnetic unit 82 repels both the first magnetic unit 73 and the second magnetic unit 74 to prevent the third magnetic unit 82 being attracted by the first magnetic unit 73 or the second magnetic unit 74 while moving along the axial direction of the sensing rod 60.
When the conventional liquid density sensor is in use, the sensing rod 60 is immersed in the liquid with the sensing module 50 exposed out of the liquid. Since the first magnetic unit 73 is mounted near the floating part 72, the first magnetic unit 73 can be horizontally aligned with the surface of the liquid. The controller 51 of the sensing module 50 outputs a pulse signal to the sensing line 61. When the pulse signal passes a position at which the first magnetic unit 73 is disposed, an induced signal is generated at the position due to a magnetic field of the first magnetic unit 73. When the controller 51 receives the induced signal, the position of the first magnetic unit 73 can be located based on a time difference between outputting the pulse signal and receiving the induced signal, thereby further calculating a liquid level of the liquid.
Furthermore, the liquid will flow into the housing 71 through the first openings 711. The second floating ball 80 will stay at a position between the first magnetic unit 73 and the second magnetic unit 74 due to a force balance among the buoyant force, weight of the second floating ball 80 (gravity), and magnetic repulsion between the third magnetic unit 82, the first magnetic unit 73 and the second magnetic unit 74. The controller 51 can respectively obtain positions of the first magnetic unit 73, the second magnetic unit 74 and the third magnetic unit 82 by using the pulse signal as disclosed above, and further obtain a distance between the first magnetic unit 73 between the third magnetic unit 82, and a distance between the second magnetic unit 74 and the third magnetic unit 82. Then, the controller 51 can obtain a density of the liquid based on the obtained distances.
With further reference to FIG. 9, a vertical axis represents the density of the liquid (kilogram/cubic centimeter), and a horizontal axis represents the distances between the third magnetic unit 82, the first magnetic unit 73 and the second magnetic unit 74 (inches). If the liquid has a relative high density, the second floating ball 80 goes up due to an increasing of the buoyant force, thus, the third magnetic unit 82 approaches the first magnetic unit 73 and leaves the second magnetic unit 74. On the contrary, if the liquid has a relative low density, the second floating ball 80 goes down due to a decreasing of the buoyant force, thus, the third magnetic unit 82 leaves the first magnetic unit 73 and approaches the second magnetic unit 74. The controller 51 calculates the density of the liquid based on two characteristic curves shown in FIG. 9.
However, the third magnetic unit 82 repels both the first magnetic unit 73 and the second magnetic unit 74, and thus, the two characteristic curves shown in FIG. 9 are both nonlinear. Calculations of nonlinear characteristic curves are very difficult and complicated, that is, the density of the liquid must be calculated through complex calculations, otherwise calculation errors easily occur and decrease accuracy of the sensing result.